1. Field of the Invention
The present invention relates to an antenna for use with polarized waves, and more particularly to an antenna for use with two orthogonal polarized waves which is provided with a tilting function.
2. Related Art
An antenna for use with two polarized waves has been developed which can transmit and receive two orthogonal polarized waves, such as a horizontally polarized wave along with a vertically polarized wave.
As an example, Japanese Patent Laid Open No. Hei 5-22025 discloses a structure of a parallel-plate slot antenna for use with two polarized waves. This antenna propagates radio waves in two orthogonal directions (horizontal and vertical directions in the plane of the paper) in the parallel-plate slot antenna, and radiates two polarized waves. More specifically, a pair of slots for radiating a horizontally-polarized wave and a pair of horizontal slots for radiating a vertically-polarized wave are formed in the upper metal sheet. The horizontally-polarized-wave radiating slots are coupled with a wave traveling in the horizontal direction in the parallel-plate waveguide, while the vertically-polarized-wave radiating slots are coupled with a wave traveling in the vertical direction in the parallel-plate waveguide.
By providing a plate antenna with two pairs of orthogonal slots, it is possible to transmit and receive both a horizontally-polarized wave and a vertically-polarized wave. However, when such a plate antenna is mounted on a specified flat surface (the roof for example) of a vehicle and used as a CS (communications satellite) antenna, it is necessary to tilt the beam from the plate antenna to the direction of the CS, but this is not possible with such an antenna.
FIGS. 8A and 8B show the relationship between the structure of the conventional plate antenna and the traveling directions of two orthogonal TEM waves and the beam tilting directions. FIG. 8A shows a case of a TEM wave traveling in the horizontal direction (left-right direction in the diagram), while FIG. 8B shows the case of the TEM wave traveling in the vertical direction (up-down direction). In FIGS. 8A and 8B, reference numeral 100 denotes a pair of slots for horizontally-polarized-wave radiation, and 200 denotes a pair of slots for vertically-polarized-wave radiation. In FIG. 8A, the pair of horizontally-polarized-wave radiating slots 100 are coupled with the horizontal TEM wave, so that an electric field E occurs in the direction as shown, Therefore, when the plate antenna is tilted in the traveling direction of the wave, the radiation is vertically polarized in the direction of this electric field E. On the other hand, in FIG. 8B, the pair of vertically-polarized-wave radiating slots 200 are coupled with the vertical TEM wave, so that an electric field E occurs in the direction as shown. Therefore, when the plate antenna is tilted in the direction of the wave, the radiated field is vertically polarized in the same way as in the case of FIG. 8A. In the conventional plate antenna, only electric field vectors parallel to the traveling direction of the TEM wave are produced, so that electric field vectors in two directions are produced. Therefore, only vertically polarized waves can be transmitted and received by tilting the plate antenna, and two polarized waves cannot be handled by a conventional plate antenna, which is a significant problem.
The present invention has been made to solve the problems with the art as explained above and has an object of providing a plate antenna which can handle two polarized waves and can also be tilted.